1. Field of the Invention
The present invention concerns a device and a method to control an electron beam emanating from an emitter of electrons for the generation of x-ray radiation, in particular for the modulation of x-ray radiation. The invention moreover concerns an x-ray tube having such a device.
2. Description of the Prior Art
In the use of x-ray radiation for imaging in medical engineering, there are various application cases in which modulation of the x-ray radiation or the radiation power within different time periods is desirable. For example, in x-ray computed tomography, particularly in the acquisition of 2D x-ray projections of measurement subjects that are not rotationally symmetrical, the x-ray radiation is matched to the respective body cross section that is to be exposed.
A further application case for modulation of x-ray radiation in x-ray computed tomography is in computed tomography apparatuses with two x-ray systems that are arranged on the rotating part of the gantry, offset by approximately 90° relative to one another. In order to avoid x-ray scatter radiation generated by the operation of the x-ray source of the other x-ray system from being detected with the x-ray detector of the one x-ray system, the emission of x-ray radiation by the x-ray source of the other x-ray system should be suppressed during the readout of measurement data of the x-ray detector of the one x-ray system. The modulation of the x-ray radiation here is achieved by a temporary deactivation of the x-ray radiation or a temporary suppression of x-ray radiation.
The modulation of the x-ray radiation for the most part ensues by a corresponding operation of the x-ray tube generating the x-ray radiation, wherein the heating power of the thermal electron emitter that is used to emit electrons is preferably varied to generate and block the electron beam. The fastest response time of the x-ray tube, or of the electron emitter of the x-ray tube, is accordingly limited by the thermal inertia of the electron emitter. A problem with the technique of varying the heating power, due to the thermal inertia, for example with regard to the aforementioned example pertaining to a computed tomography apparatus with two x-ray systems is to suppress the emission of x-ray radiation by the x-ray source of the other x-ray system during the short readout of measurement data of the x-ray detector of the one x-ray system, and to immediately apply x-ray radiation with the x-ray source of the other x-ray system again after the readout.
A device to generate x-ray radiation that has a cathode electrode, a grid electrode, a focus electrode, an anode and a voltage splitter formed by ohmic resistors is described in US 2004/0114722 A1. The voltage splitter divides a tube voltage applied to the anode in order to generate a focus voltage to be applied to the focus electrode.
A device for fast dose modulation of x-ray radiation is known from WO 2008/155715 A2, in which an electron beam for generation of x-ray radiation which should be used to expose a subject strikes a first region of an anode and in which the electron beam is deflected by a deflection means toward a second region of the anode if no subject should be exposed.
In U.S. Pat. No. 4,104,526 a device is described that has a cathode to generate an electron beam, an anode to generate x-ray radiation and a control screen to modulate or to suppress the electron beam. Moreover, the device has means to detect the anode current as a measurement unit for the current generation of x-ray radiation. This anode current is used to control the potential difference between the cathode and the control screen.